Lists and arrays can both be used to store ordered collections of data. Both have their strengths and weakness which we shall discuss in a later post.

Previously we showed you how to generate Pascals Triangle and in that Java example we used arrays to represent each row of the triangle.

The following code shows what the code would look like if we replace the use of arrays with List’s.

package com.learnjava.math;

import java.util.ArrayList;
import java.util.List;

/**
 * Write a Java program that calculates the 
 * first 10 lines of a Pascals Triangle
 * 
 * @author https://learn-java-by-example.com
 *
 */

public class PascalTriangleUsingList {

	public static void main(String[] args) {
		
		int n = 10;
		
		List<Integer> row = new ArrayList<Integer>();
		
		for (int i=0; i<n; i++) {
			
			// Calculate next row
			
			row = pascalRow(row);

			// Output row
			// First add some padding so triangle is centered
			
			for (int j=0; j< n - i; j++) {
				System.out.print(" ");
			}
			
			// Now output the row values
			
			for (int j=0; j< row.size(); j++) {
				System.out.print(row.get(j)+" ");
			}
			System.out.println();
			
		}
	}

	public static List<Integer> pascalRow(List<Integer> previous) {
		
		// Row is 1 element longer than previous row
		
		List<Integer> row = new ArrayList<Integer>(previous.size() + 1);
		
		// Initialise the list
		// Unlike arrays, a list is empty until we add values to it
		
		for (int i=0; i<previous.size()+1; i++) {
			row.add(0);
		}

		// First and last numbers in row are always 1
		
		row.set(0, 1);
		row.set(row.size() - 1, 1);
		
		// The rest of the row can be 
		// calculated based on previous row
		
		for (int i = 1; i< row.size()-1; i++) {
			row.set(i, previous.get(i-1) + previous.get(i));
		}
		
		return row;
	}
}